Radar-based position finding systems are frequently used in motor vehicles in connection with driver assistance systems, for example in so-called ACC (adaptive cruise control) systems, which allow automatic distance regulation to a preceding vehicle, or in predictive safety systems (PSS), which are used for the purpose of recognizing an imminent collision and initiating automatic measures to avoid the collision and/or at least mitigating the consequences of the collision.
The radar sensors used are typically long-range, both distance-resolving and also angle-resolving radar sensors (LRR; long-range radar), which operate at a frequency of approximately 76 GHz, or distance-resolving short range sensors (SRR; short range radar), which operate at a frequency in the range of approximately 24 GHz, or combinations of these two sensor types. Using two LRR sensors, whose position finding directions deviate from the longitudinal axis of the vehicle towards opposite sides, so that the left and right roadway edges may be monitored better and a higher position finding frequency is achieved in the overlap area on the roadway, has also been suggested.
The angle-resolving LRR sensors deliver information about the distance and the relative velocity of the positioned objects and about their azimuth angle, from which, on the basis of the distance, the approximate lateral position of the object, i.e., its location in the direction transverse to the longitudinal axis of the vehicle, may then be calculated.
For various assistance functions, it would also be desirable to have information available about the approximate width of the positioned objects, so that, for example, in a PSS system it may be better decided whether or not it is possible to drive around the object.
In the typical position finding systems, such a width estimation is not possible with satisfactory precision, however, not even if the position finding system has an angle-resolving LRR sensor. One significant reason for this is that the radar signal reflected from an object, for example the rear of a preceding vehicle, is not received uniformly from all points distributed over the width of the object, but rather is localized on one or a few reflection points, at which the perpendicular of incidence of the reflecting surface is coincident with the line of sight of the radar sensor. Depending on the spatial relationship between the radar sensor and the object, the (main) reflection point may “jump” unpredictably between various points on the rear of the object, which also restricts the precision and reliability when determining the lateral position of the object.
German Patent No. DE 103 36 638 describes a driver assistance system for motor vehicles, in which the positioned objects are classified according to their estimated width. However, no special method is specified for estimating object width.
Radar-based position finding systems having multiple 24 GHz sensors are described in German Patent Nos. DE 101 60 299 and DE 102 60 855, in which the width of the objects is estimated on the basis of characteristic patterns in the distance data measured by the various sensors. Inter alia, the circumstance is exploited that in the case of a broad, flat rear, for example, of a truck, all radar sensors situated adjacent to one another measure essentially the same distance, while the distance data of the various sensors are distributed in a characteristic way in the event of one or more localized objects.